The present disclosure relates to a duplexer and a front-end circuit. In communication devices, such as cellular phones, a duplexer which separates a transmit signal and a received signal from each other is used when signals are transmitted and received via one antenna. International Publication No. 2007/102560 discloses the following duplexer, for example. In this duplexer, a transmit filter that allows a signal of a transmit frequency band to pass therethrough and a receive filter that allows a signal of a receive frequency band to pass therethrough are each constituted by a ladder filter. In such a duplexer, a leakage of a signal from the transmit filter to the receive filter may occur. It is thus desirable to improve the isolation characteristics between the transmit filter and the receive filter. In this duplexer, a first wiring connected between an antenna terminal and a ground and a second wiring connected between a resonator included in the ladder filter and a ground are electromagnetically coupled with each other so as to improve the isolation characteristics.